1. Field of the Invention
This invention relates to the fabrication of domain reversals in a predetermined pattern on a ferroelectric for the purpose of forming an optical wavelength converter element having periodically segmented domain reversals.
1. Description of the Prior Art
A proposal has already been made by Bleombergen et al. in Physics Review vol. 127, No. 6 in 1918 (1962), in which the wavelength of a fundamental wave is converted into a second harmonic wave using an optical wavelength converter element with regions (domains) where the directions of spontaneous polarization of a ferroelectric, possessing the non-linear optical effect, are periodically inverted.
In this method, the fundamental wave can be phase matched with the second harmonic wave by setting the period .LAMBDA. of the domain reversals to be an integral multiple of the coherence length .LAMBDA. which is given by EQU .LAMBDA.c=2.pi./{.beta.(2.omega.)-2.beta.(.omega.)} (1)
where .beta.(2.omega.) designates the propagation constant of the second harmonic wave, and 2.beta.(.omega.) represents the propagation constant of the fundamental wave. When wavelength conversions are effected using the bulk crystal of a nonlinear optical material without periodically segmented domain reversals, a wavelength to be phase-matched is limited to the specific wavelength inherent to the crystal. However, in accordance with the above described method, a phase matching (that is, a socalled pseudo phase matching) can be realized efficiently by selecting a period .LAMBDA. satisfying the condition (1) for an arbitrary wavelength.
Examples of known fabrication methods for such periodically segmented domain reversals include
1) the method proposed by K. Yamamoto, K. Mizuuchi, and T. Taniuchi in Optics Letters. Vol. 16, No. 15, pp. 1156 (1991) wherein the -z surface of LiTaO.sub.3 is peridocially subjected to proton exchanges, and a resultant structure undergoes a heat treatment around the Curie temperature; PA1 2) the method proposed by H. Ito, C. Takyu, and H. Naba in Electronics Letters, Vol. 27, No. 14, pp. 1221 (1991), wherein electron beams are directly radiated onto the -z surface of unipolarized LiTaO.sub.3 or LiNbO.sub.3 at room temperature; and PA1 3) the method wherein resists which act to cut off electric charges are laid in a predetermined pattern on the surface of unipolarized LiTaO.sub.3 or LiNbO.sub.3, or the like, and then electric charges are radiated onto the entire surface by corona electrical discharging techniques.
The technique in which a charged particle beam is radiated onto a ferroelectric can be effected in a relatively simplified manner as seen from the above described second and third,techniques. However, this technique has admitted drawbacks in that domain reversal is difficult to achieve, and the reversal of domains is liable to be indefinite in the vicinity of the surface of a ferroelectric.
With such an indefinite reversal of domains as mentioned above, when the foregoing optical wavelength converter element is formed using a ferroelectric having indefinite domain reversals, no periodically segmented domain reversals will be produced, or it will be impossible to effect an effective wavelength conversion due to the indefinite period of the resultant domain reversals.
In addition, with the indefinite reversal of domains in the vicinity of the surface of a ferroelectric, when an optical waveguide type optical wavelength converter element is formed using such a ferroelectric, there arises a particular problem. Namely, because of the fact that an optical waveguide is formed in the vicinity of the surface of the ferroelectric, even though domain reversals are formed accurately in areas other than the areas adjacent to the surface, either no periodically segmented domain reversals are formed in the optical waveguide where domain reversals are expected to achieve, or the period of the domain reversals becomes inaccurate. With the use of such an optical waveguide type wavelength converter element, it is, as a matter of course, impossible to achieve an efficient wavelength conversion.